For achieving goals of being low-voltage, low-noise and low-electromagnetic interference (EMI), nowadays a liquid crystal display (LCD) panel mostly uses a differential signal to transfer data. The differential signal interface can be a low voltage differential signaling (LVDS) interface, a mini-low voltage differential signaling (mini-LVDS) interface or a reduced swing differential signaling (RSDS) interface.
FIG. 1 illustrates a circuit diagram of a signal connection between a conventional display control circuit and an LCD panel. A display control circuit 10 can be an integrated circuit (IC) mounted on a circuit board (not shown). The display control circuit 10 comprises a digital region 150 and an analog region 100. The analog region 100 comprises a bandgap voltage reference circuit 112, an operation amplifier 114, a current mirror 116, a transistor M1, an adjustable current generator 118 and an output driver 120. The digital region 150 comprises a processing circuit 152 for processing an image signal (not shown) to generate a data signal to be outputted at the output driver 120.
Generally speaking, a differential output pair of the output driver 120 can output a differential signal to an LCD panel 250. Therefore, the LCD panel 250 requires a panel resistor (Rpanel) to receive the differential signal. The data from the display control circuit 10 to the LCD panel 250 is recognized according to a voltage value on the panel resistor (Rpanel). For the same reason, the display control circuit 10 has N output drivers to output N differential signals to the LCD panel 250, and hence N panel resistors (Rpanel) are needed on the LCD panel 250 to receive such N differential signals.
Take the LVDS interface for example. Resistance of 100 ohms is required for the panel resistor (Rpanel), and a voltage swing of 350 mV is required on the panel resistor (Rpanel). Accordingly, in order to have the voltage swing on the panel resistor (Rpanel) reach 350 mV, the display control circuit 10 has to output a current of 3.5 mA (350 mV/100 ohm) exactly.
In general, the bandgap voltage reference circuit 112 provides a bandgap voltage (VBG), which is stable and not varied by manufacturing process, temperature and power voltage. The bandgap voltage is inputted to a positive input end of the operation amplifier 114, and a negative input end of the operation amplifier 114 connects to a first input/output (I/O) pin 12 of the display control circuit 10. Further, the drain of the transistor (M1) connects to a first end of the current mirror 116, the gate of the transistor (M1) connects to an output end of the operation amplifier 114, and the source of the transistor (M1) connects to the first I/O pin 12 of the display control circuit 10. The first I/O pin 12 couples to ground through an external precision resistor (Rp).
Obviously, during a normal operation of the operation amplifier 114, the voltage on the first I/O pin 12 of the display control circuit 10 is the bandgap voltage (VBG). Thus, a first current (I1) on the external precision resistor (Rp) is (VBG/Rp). The first current (I1) is outputted from the first end of the current mirror 116. Meanwhile, a second end of the current mirror 116 outputs a reference current (Iref), which is proportional to the first current (I1) and can be viewed as an accurate current.
The processing circuit 152 outputs a current control signal to the adjustable current generator 118 for controlling a multiple (M) of the adjustable current generator 118, such that a current of precisely 3.5 mA is outputted from multiplying the reference current (Iref) by the multiple (M). The output driver 120 receives the data signal output from the processing circuit 152. According to the data signal, the differential signal is driven by a 3.5 mA output from the adjustable current generator 118 to the panel resistor (Rpanel) on the LCD panel 250 via a second I/O pin 14 and a third I/O pin 16.
A connection 200 through the second I/O pin 14 and the third I/O pin 16 to the panel resistor (Rpanel) comprises a trace, a connector and a cable on the circuit board, and a connector on the LCD panel 250.
To obtain the accurate current, the conventional display control circuit 10 requires the first I/O pin 12 coupling to the external precision resistor (Rp) on the circuit board.